First, to export and import MeSince certificate to other email client is a hard work, we recommend you use MeSince as your default email client, if you need some special feature that MeSince don’t have, just let us know, we will try hard to meet your need in the future release.
For MeSince Windows version, encrypting certificates and signing certificates are securely stored in the Windows certificate store that is unexportable.
MeSince Windows version will automatically install the MeSince Root Certificate to the "Trusted Root Certification Authorities", and the user certificate is also automatically installed in the Windows user certificate store, so Office Outlook users can automatically use the Outlook to encrypt and decrypt email without any settings, just need to choose "signing" and/or "encrypting" when sending email using Outlook.
MeSince Android and iOS version don’t provide the certificate export function. Again, the export and import certificate to other email client is a hard work, no need to waste your time to do that, just use MeSince.